Televisions with cathode ray tubes (CRTs) create an image by sweeping an electron beam across a phosphorescent screen. After starting from the upper left hand corner and sweeping horizontally to the right edge of the screen, the beam is returned to the left edge, to start sweeping a new horizontal line beneath the previous line. During its return to the left edge of the screen, the beam is cut off to blank out the screen This scanning process is repeated 262 times to complete an entire "field". Typically, a second field is generated so that its scan lines lie in-between those of the previous field, the two fields defining a complete frame, which has 525 scan lines altogether. The process of scanning one field within the other is referred to as "interlacing" or "interleaving". The frames are repeated sixty times a second to avoid flicker and to give the appearance of a continuous moving image.
Fiber optics are thin plastic strands that carry light. One well-known form of fiber optic has an inner core and outer cladding which have different indexes of refraction. As a result, light introduced into a strand at one end is refracted so that it stays within the strand as it travels along its length to the other end.
Light emitting diodes (LEDs) are semiconductor illuminating devices which are also well known. LEDs can be made to emit light in a variety of colors and the intensity of this light can be controlled by the amount of electrical current passed through the device. LEDs can be switched on and off very quickly, but are too expensive to be made into a large color display for a television, which might require over 450,000 LEDs.
Liquid crystal displays (LCDs) do not produce light, they pass or block it under control of an electrical signal. The light entering the display is first passed through a polarizing filter and then through the LCD material. With a negative transmissive type LCD, the application of an electric field to the LCD material causes its molecules to be oriented to form another polarizer so the polarized light will be transmitted. When the field is removed, the polarization of the LCD material changes so that the light is blocked.
LCDs are inexpensive and have been used in small portable televisions. However, they have a poor gray scale and require additional light for color displays.
LCDs have been used to form a shutter assembly to allow light in selected areas to pass while suppressing other areas. However, the liquid crystal is not fast enough to allow a single picture element (pixel) of light through at a time to reproduce television horizontal sweep speeds. However, it can respond at vertical line frequencies.
Three major technologies available for displaying highinformation content from television signals are backlighted liquid crystal displays, gas plasma displays and electroluminescent displays.
Twisted-nematic liquid crystal displays are both voltage-and time-dependent and are limited by the response time of their materials. As a result, active-matrix liquid crystal displays were developed to improve the response time, the contrast and the angle viewability. However, active-matrix liquid crystal displays require very high resolution photo lithography steps and so are both more complicated and expensive to manufacture than are twisted-nematic liquid crystal displays.
Super twist, backlit LCDs are suitable for applications that can tolerate slower response time and narrower viewing angles. Those applications demanding high contrast, wide viewing angles and fast response rely on the other technologies, e.g. gas plasma and electroluminescent displays, which are much more expensive to manufacture.
For large panel applications, back-lit liquid crystal displays require a large amount of power, thereby shortening the battery life of the battery used to power the illumination of the display.
It would therefore be desirable to use liquid crystal displays in a video display which requires less power for illumination to achieve the same or better illumination than existing video displays. It is also desirable to use such a video display for television applications such that there is no compromise in performance despite the slower response time of liquid crystal displays.
In accordance with the present invention, a high resolution video display for textual and graphic information includes four fundamental components: a horizontally swept light source, light guides, a shutter array, and electronic driving circuitry. The light source is operable, under control of the driving circuitry to produce light which sweeps horizontally at a controllable rate. The light source may include different colors. The light guides receive the light from the light source and convert it to a vertical column of light which is one picture element wide. The shutter array comprises a plurality of elongated horizontal shutters positioned in vertical alignment so as to intercept the light from the light guides. The electronic driving circuitry responds to an electrical signal (e.g. a television signal) and produces a first control signal which sweeps the light source at the horizontal scan rate and a second control signal which opens and closes the shutters at the vertical scan rate. As a result, the shutters need only be opened and closed at a relatively slow speed corresponding to the vertical scan rate.
In an illustrative embodiment of a color display, a horizontal sweep is formed by a linear arrangement of groups of three different primary color light sources illuminating vertical optical collimators. By selecting combinations of different color light sources, the columns may be given a particular color of light at a television horizontal scan speed.
The optical light guides are preferably in the form of collimators, each having a transparent front surface which has the width of a pixel. A plurality of these guides are arranged side-by-side to form the display. The optical guides are formed so as to internally reflect and/or refract light so the light can escape out of the light guides only through the front surface.
The light sources may provide either monochrome or multiple color. The light sources may be light emitting diodes (LEDs), lasers, vacuum tube light emitters, or incandescent or fluorescent lamps. LEDs are the preferred light sources.
Movable mirrors, e.g. rotatable mirrors or piezoelectric flapper mirrors, can be arranged between the light sources and the collimators to reflect the light rays from the light sources to each of the entrance surfaces of the collimators. The mirrors could also direct the light directly to the shutter assembly.
The shutters may be elongated linear liquid crystal display elements formed one above the other. These may be of the twisted-nematic, N cap or other type.
When LEDs and LCDs are used, the electronic circuitry drives the groups of LEDs to illuminate the collimators or light guides and sequentially allows the light to pass through the apertures of the LCD shutter assembly. The electronic circuitry processes video chromatic input signals so that the intensity of the light emitted from the LEDs can be varied in response thereto and the light from different colored LEDs of each group is combined together to produce any color called for by the input signal. Thus, the groups of LEDs are typically actuated in sequence to provide a horizontal sweep of the display and the apertures of the LCD shutter assembly are actuated in sequence to provide a vertical sweep of the display.
In a preferred embodiment, the collimators may have two internal mirrors. One mirror is convex and the other mirror is concave. Preferably, the convex mirror is parabolic and the concave mirror is hyperbolic so that the light rays reflecting off the hyperbolic mirror will pass through the front surface of the collimator parallel to each other and perpendicular to the front surface.